1. Technical Field
The present invention relates to a printed circuit board and more particularly to a technique and product for fabricating a printed circuit board so as to prevent the flow of molten solder into a via during the process of wave soldering the bottom surface of the circuit board.
2. Related Art
A printed circuit board typically accommodates a surface mount device such as a ball grid array (BGA) module. A BGA module comprises an array of solder balls located on the underside of the BGA module. The top surface of a printed circuit board contains a landing area comprising a corresponding array of top pads. When a BGA module is installed on the landing area of the top surface, each solder ball rests on a top pad and is affixed to the top pad by means of a solder joint. Each such top pad is electrically connected to a via. The via is a cylindrical hole extending through the thickness of the circuit board and is lined with a material, such as copper, that acts as both an electrical and a thermal conductor. The via provides a path of electrical conduction between the BGA module and electrical circuit patterns located within internal layers of the circuit board.
After BGA modules are installed on the top surface of a circuit board, pin-in-hole components such as connectors, single in-line packages (SIPs), and dual in-line packages (DIPs) are placed on the top surface of the circuit board and are affixed by wave soldering the bottom surface of the circuit board. During the wave soldering of the bottom surface, the circuit board is transported over a molten bath of solder, wherein upward movement of solder through a chimney causes a fountain of solder to be drawn into pin holes so as to encase leads of pin-in-hole components. Unfortunately, the molten solder also travels up the vias. The hot solder within a via transfers substantial heat to its associated top pad because of a large thermal conductance between the via and the pad. Accordingly, the solder joint at the top pad is susceptible to reflow (liquidization and redistribution) from the applied heat, which may cause an open circuit. Alternatively, the heat and consequent reflow may degrade the integrity of the solder joint, thereby making the solder joint susceptible to fracture from stress during subsequent usage of the circuit board.
In order to avoid the expense and time delay associated with rework and repair due to impaired solder joints resulting from wave soldering, it is necessary to block the transfer of heat from the molten solder to the top pads. The prevailing method of inhibiting such heat transfer is to cover the via opening at the bottom surface of the circuit board with insulating material such as KAPTON.RTM. (a trademark of DuPont for a polyimide) tape. See U.S. Pat. No. 5,511,306 (Denton et al., Apr. 4, 1994) which is hereby incorporated by reference. The technique of covering the via opening has the disadvantages of adding steps to the board-fabrication process and incurring the cost of materials such as the KAPTON tape.
Thus, there is a need for an efficient and inexpensive method of preventing hot molten solder from flowing up through a via during the wave soldering process in order to inhibit heat transfer from the molten solder to a top pad.